[lvc-project] [PATCH v2 1/2] ext4: fix string copying in parse_apply_sb_mount_options()

[Fedor Pchelkin]

strscpy_pad() can't be used to copy a non-NUL-term string into a NUL-term
string of possibly bigger size.  Commit 0efc5990bca5 ("string.h: Introduce
memtostr() and memtostr_pad()") provides additional information in that
regard.  So if this happens, the following warning is observed:

strnlen: detected buffer overflow: 65 byte read of buffer size 64
WARNING: CPU: 0 PID: 28655 at lib/string_helpers.c:1032 __fortify_report+0x96/0xc0 lib/string_helpers.c:1032
Modules linked in:
CPU: 0 UID: 0 PID: 28655 Comm: syz-executor.3 Not tainted 6.12.54-syzkaller-00144-g5f0270f1ba00 #0
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:__fortify_report+0x96/0xc0 lib/string_helpers.c:1032
Call Trace:
 <TASK>
 __fortify_panic+0x1f/0x30 lib/string_helpers.c:1039
 strnlen include/linux/fortify-string.h:235 [inline]
 sized_strscpy include/linux/fortify-string.h:309 [inline]
 parse_apply_sb_mount_options fs/ext4/super.c:2504 [inline]
 __ext4_fill_super fs/ext4/super.c:5261 [inline]
 ext4_fill_super+0x3c35/0xad00 fs/ext4/super.c:5706
 get_tree_bdev_flags+0x387/0x620 fs/super.c:1636
 vfs_get_tree+0x93/0x380 fs/super.c:1814
 do_new_mount fs/namespace.c:3553 [inline]
 path_mount+0x6ae/0x1f70 fs/namespace.c:3880
 do_mount fs/namespace.c:3893 [inline]
 __do_sys_mount fs/namespace.c:4103 [inline]
 __se_sys_mount fs/namespace.c:4080 [inline]
 __x64_sys_mount+0x280/0x300 fs/namespace.c:4080
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x64/0x140 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Since userspace is expected to provide s_mount_opts field to be at most 63
characters long with the ending byte being NUL-term, use a 64-byte buffer
which matches the size of s_mount_opts, so that strscpy_pad() does its job
properly.  Return with error if the user still managed to provide a
non-NUL-term string here.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 8ecb790ea8c3 ("ext4: avoid potential buffer over-read in parse_apply_sb_mount_options()")
Cc: stable at vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin at ispras.ru>
---

v2: - treat non-NUL-term s_mount_opts as invalid case (Jan Kara)
    - swap order of patches in series so the fixing-one goes first

v1: https://lore.kernel.org/lkml/20251028130949.599847-1-pchelkin@ispras.ru/T/#u

 fs/ext4/super.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 33e7c08c9529..15bef41f08bd 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2475,7 +2475,7 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
 					struct ext4_fs_context *m_ctx)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
-	char s_mount_opts[65];
+	char s_mount_opts[64];
 	struct ext4_fs_context *s_ctx = NULL;
 	struct fs_context *fc = NULL;
 	int ret = -ENOMEM;
@@ -2483,7 +2483,8 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
 	if (!sbi->s_es->s_mount_opts[0])
 		return 0;
 
-	strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts);
+	if (strscpy_pad(s_mount_opts, sbi->s_es->s_mount_opts) < 0)
+		return -E2BIG;
 
 	fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL);
 	if (!fc)
-- 
2.51.0